Printed circuit boards (“PCBs”), multi-chip modules (“MCMs”), and similar structures having multiple integrated circuits mounted upon their surfaces are used extensively in modern electronic devices and systems. PCBs typically contain multiple conductive and dielectric layers interposed upon each other, and interlayer conductive paths (referred to as vias), which may extend from an integrated circuit mounted on a surface of the PCB to one or more conductive layers embedded within the PCB. MCMs decrease the surface area by removing package walls between chips, improve signal integrity by shortening interconnection distances and remove impedance problems and capacitances. MCMs and other similar structures typically have similar configuration and structure (e.g., a substrate comprising dielectric and conductive layers having interlayer vias). For ease of reference, all such structures shall hereafter be referred to as “boards”.
The speed and complexity of integrated circuits are increasing rapidly as integrated circuit technology advances from very large scale integrated (“VLSI”) circuits to ultra large scale integrated (“ULSI”) circuits. As the number of components per chip, the number of chips per board, the modulation speed and the degree of integration continue to increase, electrical interconnects are facing fundamental limitations in areas such as speed, packaging, fan-out, and power dissipation. MCM technology has been employed to provide higher data transfer rates and circuit densities. Conventional technologies based on electrical interconnects, however, fail to provide requisite multi-Gbits/sec clock speed in intra-MCM and inter-MCM applications.
Additionally, a printed circuit board may, in some instances, be quite large and the conductive paths therein can be several centimeters in length. As conductive path lengths increase, impedances associated with those paths also increase. This has a detrimental effect on the ability of the system to transmit high speed signals. Although the use of copper and materials with lower dielectric constant materials can release the bottleneck in a chip level for the next several years, these materials will not support interconnection speed over a few GHz even though chip local clock speeds are expected to constantly increase to 10 GHz by the year 2011. It is therefore desirable that impedances of conductive paths be minimized; in order to, for example, transmit high speed signals above the 1 Gb/sec range.
High performance materials and advanced layout technologies, such as IMPS (Interconnected Mesh Power system), focus on signal integrity to provide controlled impedance signal transmission with very low cross talk. Such an electrical interconnection provides a 10 Gb/s link over a distance less than 20 m using coaxial cable. However, coaxial cabling is bulky; therefore, it is not suitable for high density interconnection applications. Electrical interconnects operating at high frequency region have many problems to be solved such as crosstalk, impedance matching, power dissipation, skew, and packing density. However, there is a little hope to solve all of the problems. Optical interconnection does, however, have several advantages, such as immunity to the electro-magnetic interference, independency to impedance mismatch, less power consumption, and high speed operation. Although the optical interconnects have great advantages compared to the copper interconnection, they still have some difficulties regarding packaging, multi-layer technology, signal tapping, and re-workability.
The employment of optical interconnects will be one of the major alternatives for upgrading the interconnection speed whenever conventional electrical interconnection fails to provide the required bandwidth. In fact, several optical interconnect techniques, such as free space, guided wave, board level, and fiber array interconnections have been introduced for system level applications. Although these techniques successfully demonstrated high speed optical interconnection, they continue to have packaging difficulties.
Machine to machine interconnection has already been significantly replaced by optical means. The major research thrusts in optical interconnection are in the backplane and board level where the interconnection distance, the associated parasitic RLC effects, the large fan-out induced impedance mismatch jeopardize the bandwidth requirements, and interference, such as crosstalk, skew and reflection. Optical interconnection has been widely agreed as a better alternative to upgrade the system performance. For these reasons, a conductive layer having relatively high impedance can be replaced by an optical waveguide, which can transmit signals at the speed of light. Waveguides are particularly beneficial when transmitting high speed signals over relatively long distances, as signal loss is minimized.
While embedded waveguides may be formed in a board or semiconductor substrate, difficulties arise when converting electrical signals emanating from an integrated circuit, mounted on the board's surface, to optical signals within the embedded waveguide. Some conventional conversion schemes employ light emitting lasers as transmitters and photo-detectors as receivers, mounted on the upper surface of a board adjacent bonding pads/sockets, which receive integrated circuit pins. The electrical signal from an output pin of an integrated circuit is transmitted, via a conductor at or above the board's surface, to the light emanating laser; which then converts the electrical signal to optical energy. That optical energy permeates from the board surface, through several layers of the board, downward to a waveguide embedded within the board. A grating coupler is typically placed within the waveguide to receive the optical energy and directionally transmit an appropriate wave through the waveguide; eventually to be received by an optical receiver distally located from the grating. An optical receiver can be placed proximate to another integrated circuit, separate from the integrated circuit initiating the transmitted optical signal. The optical receiver can then receive the optical energy, converting it to an electrical signal to be transmitted to an input pin of the receiving integrated circuit.
Thus, using an optical waveguide enhances the speed at which signals can be transmitted between integrated circuits. However, inefficiencies in transmitting optical energy through several layers of conductive and non-conductive materials within a board limit the light-to-electrical and electrical-to-light (optoelectronic) coupling efficiency; thereby limiting high-speed signal transmission within a system.
Additionally, conventional optoelectronic interconnect systems are typically incompatible with commercial manufacturing processes utilizing boards. Consider, for example, a printed circuit board used as a motherboard within a personal computer. A motherboard manufacturer will typically, if not exclusively, use automated equipment and processes to mount desired semiconductor devices on the surface of a printed circuit board. Optoelectronic devices often require care in handling and processing that standard semiconductor devices do not. Therefore, use of conventional optoelectronic interconnect systems will either require modification of standard manufacturing processes or additional processing steps to account for the presence or addition of optoelectronic components on the board surface. Additional monetary and time costs resulting from use of conventional optoelectronic interconnect systems thus render these approaches commercially unviable
Moreover, semiconductor lasers dissipate a lot of electrical power, so generated heat can cause catastrophic failure of the laser device without proper cooling. The embedded lasers are also thermally isolated by surrounding insulators, so heat builds up and the operating temperature increases. In addition, an embedded laser cannot be replaced or repaired in a fully embedded integration. As a result, proper thermal management of the laser is pivotal. Present technologies attempt to solve this problem by using a thermal conductive heat sink assembled on top of the printed circuit board to cool down the semiconductor laser. These heat sinks are bulky and occupy real estate of the printed circuit board, which makes alignment to the optical medium, such as an optical waveguide, difficult.
As described in U.S. Pat. No. 6,243,509 issued on Jun. 5, 2001, fully embedded PCB level optical interconnects make the packaging reliable and robust. It provides not only process compatibility with a standard PCB process but also reduced footprint of PCB through fully embedding all optical components such as light sources, channel waveguides and detectors among other electrical interconnection layers. However, in this configuration, VCSEL (Vertical Cavity Surface Emitting Laser) array as a light source encounters a thermal management concern for the active region of the VCSEL arrays because it is encapsulated with thermal insulators such as polymer waveguide and bonding film (prepreg). Only the common bottom metal contact of the VCSEL array can be used as a thermal interface. The VCSEL cannot operate without proper cooling. Therefore heat management of driving such a VCSEL array is a critical issue in the fully embedded structure.
Another issue regarding electrical-to-optical transmitters disposed within a printed circuit board involves the fabrication of the reflective elements. The reflective elements are 45 degree waveguide micro-mirror couplers used to couple light into and out of the waveguides at 90 degrees. For example, the angle of the plane of the optical waveguide and the propagation direction of the light source is 90 degrees. The reflective elements or 45 degree waveguides are typically fabricated using laser ablation, oblique reactive ion etching (RIE), temperature controlled RIE, re-flow and machining. The laser ablation method is a slow process that is not suitable for the fabrication of a large number of micro-mirrors. In addition, it is subjected to lower throughput and surface damage (does not leave a smooth surface, which causes scattering losses). The oblique RIE method is limited by directional freedom, so it cannot be used if the layout is complex (e.g., different direction of micro-mirrors). The temperature controlled RIE method is free from directional freedom but the quality of the mirror depends on process and materials. The re-flow method is also subjected to lower throughput. The machining provides good surface profile; however, it is difficult to cut individual waveguide on a substrate due to the physical size of the machining tool.
Yet another issue regarding electrical-to-optical transmitters disposed within a printed circuit board involves the fabrication of the channel waveguide structure. Typically, the channel waveguide structure is fabricated using photolithography, reactive ion etching, laser ablation, imprinting or molding. The reactive ion etch (RIE) uses ionized gas to remove material where it is not protected by a mask material in a vacuum chamber. The size of the substrate purely depends on the vacuum chamber. It is relatively free from material selection because RIE is a physical removing process. The lithography uses optically transparent and photosensitive materials. Exposed or unexposed area by UV light makes the material insoluble to solvent due to the cross linking of molecule. However, there is a limitation for choosing material due to the lack of materials which have optical transparency in the interested region and photosensitivity. Hot embossing and molding are indirect fabrication techniques by means of transferring waveguide structure on the substrate. Embossing plate or cast is first fabricated using the master waveguide pattern. Once the plate or the cast was fabricated, the rest of processes are purely replication steps. Therefore, these fabrication techniques are suitable for mass production like stamping of compact disk. Laser ablation technique is similar to carving without a using chisel. Highly intensive UV laser beam removes the material of unwanted region. The motion stage which holds waveguide substrate is moved along the predefined paths. Therefore, processing time is quite long. It is a quite versatile tool for small quantities in fabrication and does not require a mask pattern. All of these processes, except for imprinting and molding have a slow process time and are not suitable for making large format optical components, such as waveguides or couplers or for use in mass production lines. The imprinting method can be used to make large scale optical waveguide layers, but the fabrication process is complex.
There is, therefore, a need for a system, method and apparatus for improved electrical-to-optical transmitters disposed within a printed circuit board using improved heat dissipation and fabrication techniques.